Medium effects on the electron-transfer processes: Reduction of aquapentaamminecobalt(III) by hexacyanoferrate(II) in ethyleneglycol-water, ethylene carbonate-water and urea-water media

Abstract

The outer-sphere electron transfer between Co(NH₃)₅OH<img src='http://www.niscair.res.in/jinfo/equation28.gif' border=0> and Fe(CN)<img src='http://www.niscair.res.in/jinfo/equation29.gif' border=0> <em>- </em>which proceeds via precursor ion-pair formation has been investigated in ethylene glycol-water (0-47.6 wt% of EG), ethylene carbonate-water (0-40 wt% of EC) at 20-35ᵒC and in urea-water (0-19.4 wt% of EG) at 25ᵒC (<em>I</em> = 0.50 mol dm^-3 NaNO₃)· The electron transfer rate constant for the ion pair (<em>k, </em>s^-1) follows the order H₂O ~ U-H₂O < EG-H₂O << EC-H₂O. The specific solvent effect on the rate constant for EGH₂O and EC-H₂O media can be account d for in the frame work of the Marcus theory by considering the ion solvent dipole interaction in the solvent cospheres of the precursor and the successor ionpairs. Both <img src='http://www.niscair.res.in/jinfo/delta.gif' border=0><em>H</em>^≠ and <img src='http://www.niscair.res.in/jinfo/delta.gif' border=0><em>S</em>^≠ decrease linearly with increase in mole fraction of EC(X_EC) while for the EG-H₂O media a broad maximum s discernible in the plot of <img src='http://www.niscair.res.in/jinfo/delta.gif' border=0><em>H</em>^≠ (or <img src='http://www.niscair.res.in/jinfo/delta.gif' border=0><em>S</em>^≠)_­versus X_EG indicating thereby that the solvent structural effects might mediate the activation process.